Parkinson's disease is a disorder which affects brain nerve cells (neurons) that control muscle movement. Accordingly, people with Parkinson's often have difficulty walking or maintaining motor motions (without trembling). Stroke victims also experience difficult or limited movement, on one or both sides of the body.
Although therapy for Parkinson's and post-stroke symptoms is available, researchers still rely on patient observation (for example by studying patient gait) to quantify problems suffered by the patient. Treatment for Parkinson's in any particular patient is therefore still an iterative process involving trial and error.